The present invention relates generally to planar lightwave circuits. More particularly, the present invention relates to a method for growing an optical waveguide core material by plasma enhanced chemical vapor deposition (PECVD) with better refractive index control.
Planar lightwave circuits comprise fundamental building blocks for the modern fiber optic communications infrastructure. Planar lightwave circuits are generally devices configured to transmit light in a manner analogous to the transmission of electrical currents in printed circuit boards and integrated circuit devices. Examples include arrayed waveguide grating devices, integrated wavelength multiplexers/demultiplexers, optical switches, optical modulators, wavelength-independent optical couplers, and the like.
Planar lightwave circuits generally involve the provisioning of a series of embedded optical waveguides upon a semiconductor substrate (e.g., silicon), with the optical waveguides fabricated from one or more silica layers, formed on an underlying semiconductor substrate. Fabrication techniques required for manufacturing planar lightwave circuits using silica are generally well known.
Core refractive index control is very critical to the planar lightwave circuit devices. For example, the center wavelength of each channel in an Arrayed Waveguide Grating (AWG) device is directly affected by the refractive index of the core. A deviation of refractive index within 0.0001 will cause the channel center wavelength to vary in the region of 0.1 nm. For a 40 channel AWG operating in the C band (1520 nmxcx9c1565 nm), the channel to channel spacing is only 0.8 nm. Therefore, the core refractive index has to be accurate to about 0.0003 across the substrate to provide a high quality AWG device.
Prior art FIG. 1 shows a cross-section view of a conventional planar optical waveguide. As depicted in FIG. 1, the planar optical waveguide includes a doped SiO2 glass core 10 formed over a SiO2 bottom cladding layer 12 which is on a silicon substrate 13. A SiO2 top cladding layer 11 covers both the core 10 and the bottom cladding layer 12. As described above, the refractive index of the core 10 is higher than that of the cladding layers 11 and 12. Consequently, optical signals are confined axially within core 10 and propagate lengthwise through core 10. The SiO2 glass core 10 is typically doped with Ge or P to increase its refractive index.
One prior art method for fabricating the core 10 of the planar optical waveguide of FIG. 1 uses a phosphorus doped silica glass layer (PSG) over the SiO2 glass bottom cladding 12. P2O5 is formed in a silica matrix during PECVD deposition, which raises the refractive index. As described above, the PSG layer is subsequently fabricated (e.g., with lithography and etch processes) into the waveguide core 10.
There are three major problems with using PSG as a waveguide core material. The first problem is that the phosphorus dopant has a tendency to accumulate and form bubbles within the PSG core layer during annealing. The bubbles comprise material defects that greatly reduce the yield of the fabrication process. One method of controlling bubble formation involves the use of multi-step deposition-anneal cycles. Although bubble formation is reduced, it is not completely eliminated. Additionally, the use of multi-step deposition-anneal cycles adds time and expense to the overall planar lightwave circuit fabrication process.
The second problem is that the P2O5 in the PSG core is much more easily etched in comparison to SiO2. This causes an uncontrollable lateral etch on the side wall of the PSG core and will significantly vary the width of waveguide, which greatly affects the device performance.
The third problem is that the refractive index of PSG decreases with increasing annealing temperature and annealing time, which is caused by phosphorus thermal migration out of the core (dopant loss). Thus, any slight annealing temperature variation will change the core refractive index. As a consequence, the center wavelength will shift in an AWG.
Another prior art method for fabricating the core 10 of the planar optical waveguide of FIG. 1 is to use Germanium as the dopant for the core layer. GeO2 is formed in silica matrix during deposition, which raises the refractive index. However, this germanosilicate glass (GeSG) core layer also experiences problems. One is its large birefingence; almost three times larger than that of PSG. Another problem is that the refractive index of GeSG increases rapidly with increasing anneal temperature.
Prior art FIG. 2 shows a graph depicting polarization dependent wavelength shift and polarization dependent loss for TE and TM propagation modes, as caused by large birefringence. As depicted in FIG. 2, a TE signal component and a TM signal component are graphed after having experienced polarization dependent wavelength shift (PDW) and polarization dependent loss (PDL), from, for example, propagation along the core 10 of the planar optical waveguide of FIG. 1. The vertical axis of the graph shows amplitude and the horizontal axis shows frequency. As described above, the difference in propagation constants for the TE and TM signal components results in a PDW wavelength shift 21 in the spectral response peak between the TE and TM signal components. This wavelength shift in turn causes a PDL loss 22.
Thus what is required is a solution that provides a predictable and stable refractive index for optical waveguide cores within a planar lightwave circuit. What is required is a solution that renders the refractive index of an optical waveguide core less sensitive to variation in the annealing procedure. What is further required is a solution that eliminates the formation of bubbles and other types of flaws within a core layer. What is required is a solution that reduces birefringence of an optical waveguide core to further reduce polarization dependence of PLC device performance. The present invention provides a novel solution to the above requirements.
The present invention is a method of depositing a germanophosphosilicate glass (GePSG) core layer for an optical waveguide structure of a planar lightwave circuit. The present invention eliminates the formation of bubbles and reduces other types of flaws within a core layer. The present invention provides a waveguide core with a predictable and stable refractive index, which is independent of variations in the anneal procedure.
In one embodiment, the present invention is implemented as a method of controlling the flow rates of two doping gases, a Ge dopant gas (e.g., GeH4) and a P dopant gas (e.g., PH3) during core layer deposition for an optical waveguide structure of a planar lightwave circuit, to make a germanophosphosilicate glass (GePSG) core layer. The GePSG core for an optical waveguide structure of a planar lightwave circuit is fabricated by PECVD such that the optical core comprises two doping phases, GeO2 and P2O5, within a SiO2 matrix. The flow rate of the Ge dopant and the flow rate of the P dopant are controlled to form the GePSG core layer having precisely determined ratios of GeO2 and P2O5. A minimum birefringence can be obtained at a specific ratio of GeO2 and P2O5.
Controlling of the flow rate for the Ge dopant and the flow rate for the P dopant is configured to reduce the formation of bubbles within the core layer. The flow rates for the Ge dopant and P dopant is configured to reduce birefringence within the core layer across an annealing temperature range for the core layer deposition and annealing process. A thermal anneal process for the core layer can be within a temperature in a range of 900 C. to 1200 C. The reduced amount of P2O5 (in comparison to a PSG-only doped core layer) enhances the profile control of the core during core layer etch. The GePSG core for an optical waveguide structure significantly reduces the refractive index sensitivity to anneal temperature.